1. Field of the Invention
The present invention relates to the field of variable gain amplifiers.
2. Prior Art
A classic single quadrant variable gain amplifier is shown in FIG. 1. The amplifier consists of a Gm (transconductance) stage comprised of transistors Q1 and Q2, emitter resistors Re and current source Iee, and a current steering stage comprised of transistors Q3 through Q6 loaded by resistors Rl (the phrase current source as used herein and in the claims is used in the general sense to designate both current source and sinks, as is common in the art).
An input signal Vin generates output currents i.sub.1 and i.sub.2 via the Gm stage, where i.sub.1 +i.sub.2 =Iee. By setting the gain control voltage V.sub.G, an appropriate percentage of the currents i.sub.1 and i.sub.2 is impressed upon the load resistors Rl, generating the output voltage Vo. The remainder of the currents i.sub.1 and i.sub.2 are shunted to the common mode supply through transistors Q4 and Q5. In the analyses to follow, the base currents of the transistors are assumed to be zero for analytical convenience.
The simple stage of FIG. 1 can generate a large variable gain range by proper choice of the range of the control voltage V.sub.G, though correct transistor area choice is critical to achieve dynamic range requirements. Also this amplifier topology can generate a large quantity of noise, especially in the high gain scenario where the majority of the currents i.sub.1 and i.sub.2 is steered to the load resistors R.sub.L. Transistors Q3 through Q6 contribute a large quantity of noise, hence these devices must be quite large, relative to the bias currents used in the stage. As the gain of the stage is decreased, lowering the signal (and bias) current of the current steering transistors Q3 and Q6, the bandwidth of the stage degrades, resulting in amplifier bandwidth that is dependent upon the gain of the stage. This is not desirable in a high bandwidth, variable gain amplifier.
Simply adding common base connected transistors to the variable gain amplifier, as shown in FIG. 2, helps the problem some, but does not solve the problem. The wide variation in bias, as well as signal current, will result in degraded bandwidth with decreasing gain. This addition also introduces non-linear effects due to the highly variable bias conditions of the common base stage, which directly impacts the input impedance of the transistors, presenting a highly variable load to the variable gain amplifier. Both the real and imaginary components of this load will change. At some gain settings, the component values will be undesirable for the current steering stage, impacting linearity and stability, generating harmonic and oscillatory spurs.